For new wireless systems such as WLAN and Wi-MAX, power amplifiers with high linear efficiencies are needed. Some sort of class AB or dynamic biasing is typically used to obtain high linear efficiencies. Other applications such as high dynamic range receivers can also benefit from dynamic biasing. Such receivers improve the input compression of a low noise amplifier (LNA).
Conventional approaches implement class AB configurations as a popular solution. Many implementations are used, most of which include external manual tuning of resistors. Such implementations have extra control ports, and can be very sensitive to process or temperature variations.
Referring to FIG. 1, an example of a circuit 10 illustrating a conventional approach is shown. The circuit 10 illustrates a traditional class AB bias circuit applied to a power amplifier transistor Q1. Many conventional class AB circuits implement a pass gate QPG, which supplies a base current IB to amplifier transistor Q1. The base current is received from the supply VCC. As input power is increased, the base current IB increases. Conventional approaches include diode temperature compensation D1 and D2, which are often added to provide stability over temperature. A resistor R1 is typically externally adjusted in order to set the quiescent current of the transistor Q1. A coupling capacitor C1 detects and integrates an RF input signal IN. The coupling capacitor C1 increases the voltage applied to the pass gate transistor QPG in order to provide more base current to the transistor Q1 when under drive.
The circuit 10 has one or more of the following deficiencies (a) needing an additional supply VCC input pin which is incompatible with a SOT89 package configuration, (b) needing manual tuning of the resistor R1 in order to set the bias current of the transistor Q1 that is not very stable, and (c) providing a detection circuit that is open loop.
Referring to FIG. 2, a circuit 20 illustrating another conventional approach is shown. The circuit 20 illustrates a variation of the popular class AB bias. The circuit 20 uses a current mirror implemented as a mirror transistor QMIRR. The transistor QMIRR sets the quiscent bias. The quiscent bias adjustment allows substantially less process sensitivity compared to the bias network 10 of FIG. 1. Similar to the circuit 10, the circuit 20 uses a pass gate transistor QPG for the supplying base current IB to the amplifier transistor Q1 under RF input drive.
The circuit 20 also uses a separate supply voltage/input pin for operation of the pass gate transistor QPG. Unlike the circuit 10, the circuit 20 does not include a feed-forward coupling capacitor to enhance the class AB action. The circuit 20 also illustrates a series base resistor RBB. The size of the base resistor RBB determines the amount of class AB action that is attainable. The circuit 20 also uses an open loop configuration.
It would be desirable to implement a compact class AB dynamic feedback bias that may (i) be implemented without (a) external resistor tuning and (b) additional dedicated control ports, (ii) be implemented with a two terminal gain block, such as a Darlington feedback amplifier, and/or (iii) be implemented without being sensitive to process and/or temperature variations.